First appearance of type II neurons during ontogenesis in the spiral ganglion of the rat. An immunocytochemical study.

Ontogenesis of spiral ganglion in the rat was studied using antibodies to three subunits of neurofilaments (NFs): NF 68 KDa, NF 160 KDa and NF 200 KDa. The expression of immunoreactivity was examined with 3 immunocytochemical methods: indirect immunofluorescence, peroxidase-antiperoxidase and avidin-biotin complex. Aim of the study was to detect the time of differentiation of the spiral ganglion type II neurons. At 16 and 18 days of gestation, most neuron cell bodies express immunoreactivity to only two NF subunits: NF 68 and NF 160, but at birth they react with the antibodies to all 3 subunits albeit weakly. Nevertheless, a small population (about 7%) of nerve cells that strongly reacts against all 3 NF subunits emerges in the basal turn, already at 20 days of gestation. Two to 3 days after birth, the strongly stained cells are dispersed throughout the entire ganglion. The intensity of their reaction to the NF antibodies is similar to that seen in the adult animal. The strong immunoreactivity of this selective neuronal population suggest, that they correspond to the type II spiral ganglion neurons. Our results imply that the differentiation between the type I and the type II of spiral neurons in the rat occurs perinatally.